Fenthion is an organophosphate
compound. These compounds act as an inhibitors of cholinesterase enzymes, which are
involved in normal transmission of nerve impulses within the nervous system. (B36.39.w39)

In mosquitoes, fenthion is toxic to both
the adult and immature forms (larvae). (W180.Feb02.WNV2)

Once used extensively in the U.S. for controlling
intestinal worms, fenthion no longer has FDA approval due to poisoning deaths. (W180.Feb02.WNV2)

While it is effective as an insecticide, it is
also moderately toxic to mammals, and highly toxic to birds.

Based on its high toxicity to birds, fenthion is
used in various parts of the world for weaver bird control. Pest control operators have
used it to control pigeons around public buildings, as well. For bird control, use is made
of fenthion's contact action and its ready absorption through the skin. It is applied as a
paste to roosting areas when utilized for such purposes. (W180.Feb02.WNV2)

It is classified by the U.S. Environmental
Protection Agency (EPA) as a Restricted Use Pesticide (RUP) due to the special handling
warranted by its toxicity. All bird control products, as well as non-domestic,
non-granular formulations of 70% and greater are RUPs. RUPs may be purchased and used only
by trained certified applicators. Fenthion may not be used on food crops in the USA.
Labels for products containing fenthion must bear the Signal Word "WARNING". (W180.Feb02.WNV2)

Acute effects of fenthion are similar to those
caused by other organophosphates, but may take somewhat longer to develop.

Fenthion is of sufficiently low toxicity it has
been investigated as an agent against insect parasites in animals (e.g., dogs
(Canis
familiaris - Domestic dog)).

Symptoms of acute exposure to organophosphate or cholinesterase-inhibiting
compounds may include the following: numbness, tingling sensations, incoordination,
headache, dizziness, tremor, nausea, abdominal cramps, sweating, blurred vision,
difficulty breathing or respiratory depression, and slow heartbeat. Very high doses may
result in unconsciousness, incontinence, and convulsions or fatality.

Veterinary clinic workers who did not use skin
protection when applying a 20% topical application to dogs experienced symptoms ranging
from tingling and numbness of the hands and feet to generalized weakness and shooting
pains.

Other possible effects are similar to those caused
by the other organophosphates.

Single injections of 40 or 80 mg/kg of fenthion
into the abdominal cavities of pregnant female mice caused poisoning in the developing
fetuses, particularly when administered on days 10 to 12 of gestation.

Fetuses were injured primarily by dosages that
caused toxicity in the maternal mouse.

No influence was seen on reproduction in other
three-generation studies of mice.

Some reduction in fetal weight occurred, but no
defects were found in mice that were given intraperitoneal doses of up to 80 mg/kg of
fenthion in single-day or 3-day periods during the period of gestation in which organs are
formed.

No teratogenic effects were seen in five
generations of mice that drank water containing 60 mg/L fenthion.

Other tests on mice and rats did not show
teratogenic effects from fenthion.

Animal tests and human use experience have
identified that target organs affected by fenthion exposure include the central nervous
system, peripheral nervous system, and the heart. (W180.Feb02.WNV2)

Fate in humans and animals:

In animals, fenthion is quickly absorbed into the
bloodstream through the digestive tract, lungs, and skin, and is systemically
distributed.

Fenthion is eliminated through the urine and the
feces.

A single dose of the insecticide has prolonged
action, suggesting that much of it is stored in body fat and later released for
metabolism.

Fenthion and its metabolites were found in the fat
of steers slaughtered 3 days after dermal application of
fenthion.

When cows (Bos
taurus - Domestic cattle) were given a
dermal application of 9 mg of fenthion per kilogram, 45 to 55% of the dose was excreted in
the urine, 2.0 to 2.5% in the feces, and 1.5 to 2.0% was recovered in the milk.

Breakdown in soil and groundwater:
Fenthion has moderate persistence in soil, with an average field half-life of 34 days
under most conditions. In soil, residues of fenthion may persist for approximately 4 to 6
weeks. (W180.Feb02.WNV2)

It adsorbs fairly strongly to soil particles, and
so is not likely to move (or leach) through the soil. (W180.Feb02.WNV2)

Breakdown in water: In one study
of its persistence in water, 50% of applied fenthion remained in river water 2 weeks
later, while 10% remained after 4 weeks. (W180.Feb02.WNV2)

It is more rapidly degraded under alkaline
conditions. (W180.Feb02.WNV2)

Breakdown in vegetation: Fenthion
is phytotoxic (or harmful to plants) to American linden, Hawthorn and sugar maple trees,
and to certain rose varieties. It is not considered phytotoxic when used at recommended
rates, although injury has occurred in certain varieties of apples and cotton.

Plant foliage should not be sprayed when
temperatures exceed 90° F.

Only about 10% of applied fenthion remained on
rice plants after 6 hours. Almost half of the activity was found in the rice bran, 6.5%
was in the husk, and 14.7% was in polished rice.

Water soluble metabolites were found 14 days after
fenthion application to rice plants.

Acute symptoms of fenthion poisoning in birds
include tearing of the eyes, foamy salivation, lack of movement, tremors, congestion of
the windpipe, lack of coordination in walking, and an abnormally rapid rate of breathing
or difficult breathing.